The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling-down also produces a relatively high power dissipation value, which may be addressed by using low power dissipation devices such as complementary metal-oxide-semiconductor (CMOS) devices.
The scaled down semiconductor ICs have been used in a variety of applications. In some applications, the ICs may include both noisy devices such as high speed digital circuits and noise-sensitive devices such as analog or radio frequency (RF) circuits. When a noisy device and a noise-sensitive device are placed in a substrate together, particularly if they are placed in proximity of each other, the noise produced by the noisy device may adversely affect the performance of the noise-sensitive device. The noisy device may generate noise due to high speed switching, which then may be coupled to the noise-sensitive device through the substrate. One approach utilizes doped isolation features to isolate the devices from each other. Although this approach has been satisfactory for its intended purpose, it has not been satisfactory in all respects. For example, it requires additional photomask (or reticle) for the implantation process, and may still suffer from noise due to a capacitor coupling effect.